This invention relates to a method and apparatus for generating an input signal for a parameter sensitive circuit, particularly, to a method and apparatus for predistorting an input signal for a tunable circuit, such as an oscillator circuit or a filter circuit, so that an output of the tunable circuit is substantially corrected for non-linearities in the tuning of the tunable circuit.
Many frequency controlled tunable circuits, such as oscillator or filter circuits have non-linear frequency output characteristics, that is, the frequency output vs. control (current or potential) characteristic is not linear. When good linearity is desired, oscillator designers typically select component types and apply resonant effects to achieve a characteristic that meets the requirement as nearly as possible. Sometimes, system designs need to be modified, when it is not possible or practical to achieve the desired linearity. Even when linearity is achievable, component and dimension tolerances can require either that components are individually selected following device tests, or that physical adjustments are made during the manufacturing process. This obviously increases production costs.
In some applications, absolute control rate can be important. Temperature dependencies can limit the final system performance, or require that an otherwise low-power system be placed in a temperature controlled environment, such as an oven.
It will be appreciated that a constant frequency tuning rate can be important for systems, such as temperature compensated crystal oscillators, to avoid degradation of frequency accuracy when the oscillator is tuned away from the conditions under which it was compensated. Such tuning may be necessary to correct for, for example, ageing of the quartz crystal, or to match the operating environment. One situation where this is particularly relevant is in retiming circuits where a local crystal oscillator tracks an intermittent input clock with a frequency that is allowed to deviate from a nominal value. To ensure a rapid recovery from loss of input clock signal, the local crystal oscillator is required to continue oscillating at the last observed frequency for extended periods.
The present invention therefore seeks to provide a method and apparatus, which overcomes, or at least reduces the above-mentioned problems of the prior art, for generating an input signal to a parameter sensitive circuit so that an output of the tunable circuit is substantially corrected for tuning non-linearities in the tunable circuit.
Accordingly, in a first aspect, the invention provides a method of generating a control signal for a parameter sensitive circuit, the method comprising the steps of receiving a first input control signal, receiving a second input signal indicative of the parameter to which the circuit is sensitive, generating a third intermediate signal from the first and second input signals, generating at least a fourth intermediate signal having a non-linear dependence on at least one of the first and third signals, and generating an output control signal for the parameter sensitive circuit, by combining at predetermined levels at least the fourth intermediate signal and one of the first, second and third signals such that the output control signal is dependent on both the first and second input signals, whose sensitivity to changes in one of the input signals is dependent on the level of the other input signal.
Preferably, at least the fourth intermediate signal comprises a polynomial function of at least one of the first and third signal on which it is dependent.
The third signal preferably comprises the sum of a constant, a constant multiple of the first input signal, and a constant multiple of the product of the first and the second input signals. The first and second input signals are preferably at least partly independent of each other, that is, neither of the signals can be determined solely from the other.
In a preferred embodiment, the first input signal is dependent on the same parameter as the second input signal. The output control signal preferably comprises the sum of signals that are polynomial functions of the third signal, but may alternatively comprise the sum of signals that are polynomial functions of the third signal and a temperature-dependent signal. In one embodiment, there is further included the step of providing at least one predetermined gain control signal for controlling the gain of any of the signals utilised to generate the output control signal, so that the sensitivity of the parameter sensitive circuit to changes in the first input signal is substantially independent of both the parameter and the level of the first input signal.
In a preferred embodiment, the parameter is temperature, but may be any other environmental parameter, such as pressure or acceleration.
In a second aspect, the invention provides a circuit for generating a control signal according to the method described above, and, in a further aspect, provides an apparatus comprising a parameter sensitive circuit and such a circuit for generating a control signal.
In a preferred embodiment, the parameter sensitive circuit comprises a tunable circuit and the output control signal of the circuit compensates for parameter sensitive variations of tuning components that are used to tune the tunable circuit.
Preferably, the output control signal of the circuit comprises the sum of signals that are polynomial functions of the third signal and a temperature-dependent signal, said temperature-dependent signal being suitable for compensating for variations of built-in potential of the tuning components.
The coefficients of the polynomial functions are preferably adjustable to compensate for non-linear tuning of the tunable circuit.
Preferably, the second input signal is dependent solely on temperature, and is suitable for compensating for residual temperature dependence of the tuning sensitivity of the tunable circuit. The first input signal preferably includes a temperature dependent component suitable for compensating for a temperature dependence of a nominal resonance frequency of the tunable circuit.
Preferably, the first input signal is generated as a sum of a signal that is dependent only on temperature, and another signal that is independent of temperature, the first input signal being suitable for setting a nominal resonance frequency of the tunable circuit to a frequency that is dependent on this temperature-independent signal, and with a predetermined temperature dependence. The temperature-dependence of the tuning rate preferably provides compensation for temperature sensitivity of other components of a phase-lock-loop incorporating the tunable circuit.
At least one component of the tunable circuit may be oscillatory and the tunable circuit preferably incorporates at least an electro-acoustic resonator.
In a further aspect, the invention may provide an integrated circuit incorporating an apparatus for generating a control signal for a tunable circuit as described above. Preferably, the integrated circuit further incorporates a memory for storing at least one of temperature dependent and non-linear parameters. The integrated circuit preferably further incorporates at least part of the tunable circuit.